1. Field of the Invention
The present invention relates to a switching device for use in a flat display panel and a method of manufacturing the same. More particularly, the present invention relates to a method of forming a polysilicon film, a thin film transistor including the polysilicon film, and a method of manufacturing the thin film transistor.
2. Description of the Related Art
Thin film transistors are used as switching devices in flat display panels such as organic light emitting diodes (OLEDs) or liquid crystal displays (LCDs). A channel region of a thin film transistor can be composed of amorphous silicon or polysilicon.
On one hand, when a channel region of a thin film transistor is composed of amorphous silicon, uniformity of the channel region can be increased. However, due to low mobility of a carrier, it is difficult to attain high speed performance. On the other hand, when the channel region of a thin film transistor is composed of polysilicon, the mobility of a carrier can be greater than in a channel region composed of amorphous silicon.
When the channel region of a thin film transistor is composed of polysilicon, the polysilicon that constitutes the channel region is formed by crystallizing the amorphous silicon. The method of crystallizing the amorphous silicon can be categorized into an excimer laser method (ELA) and a solid phase crystallization method (SPC). Of the two crystallization methods, presently, the ELA method is widely used, because the ELA method has a lower thermal budget and provides a greater field effect mobility. However, using the conventional ELA crystallization method, it is difficult to produce crystals grain sizes larger than 0.5 μm, to obtain uniform grain size and to control a position on which the crystal is formed.
Accordingly, new methods of crystallization such as a sequential lateral solidification (SLS) method, an optical phase shift mask (OPSM) method, and a pre-patterned laser beam mask (PLBM) method, have been introduced. However, these methods require a correct substrate control device and require multiple pulses of a laser beam. Therefore, there are difficulties in applying these new methods to the present process for forming thin film transistors.